4 Darbishire, Mendelian Principles of Heredity. 



25% green-seeded and 75% yellow-seeded peas. Suppose 

 we take actually 75 yellow-seeded peas (the number, of 

 course, does not matter, so long as it is large ; but 75 is 

 convenient as we shall shortly see), and plant some seeds 

 from each plant. What Mendel found was this ; 50 of 

 the 75 were hybrids because 25% of their offspring were 

 ' green ' and 75% ' yellow ' (which we will now write instead 

 of ' green-seeded ' and ' yellow-seeded '), while the remain- 

 ing 25 proved themselves to be pure yellows by breeding 

 true — by producing only 'yellows'. That is to say the 75 

 yellow peas are composed of 50 hybrids and 25 dominants; 

 and now that we have at last found out what they are, let 

 us look at the whole result of breeding from the hybrids. 

 We see immediately that : — 

 25% greens + 75% yellows 



is really represented by 

 25% pure greens 50% hybrid yellows 25% pure yellows 



or 

 25% recessives 50% hybrids 25% dominants. 



Mendel experimented with the new hybrids {i.e., the 

 children of the first hybrids) and found that they too pro- 

 duced offspring, 25% of which were green and 75% yellow; 

 and he found (though he was working with small 

 numbers : Bateson :o2 p. 57) that, for at most six 

 generations, it was a general rule that hybrids when 

 paired together gave 25% recessives, 50% hybrids, and 

 25% dominants. This phenomenon is spoken of as 

 segregation ; which consists in the despatch by the hybrids, 

 at each generation, of offspring into the dominant and 

 recessive ranks from which (so long as like mates with 

 like) there is no returning. 



If we symbolise the dominant character by Z>, the 

 recessive by R., and the hybrid by DR (and if we 



